* Yang cards a 68 to grab early one-shot lead
* British Open champion Oosthuizen opens with a 69 (Adds further quotes, detail, later finishers)
South Korean Yang Yong-eun, benefiting from a conservative strategy, birdied three of his last eight holes to take charge in the U.S. Open first round at Congressional Country Club on Thursday.
Yang, who became Asia's first male major winner after overhauling Tiger Woods to clinch the 2009 PGA Championship, carded a three-under-par 68 on the undulating Blue Course to hold the clubhouse lead.
"The last few U.S. Opens that I attended, I was over par so I'm very satisfied right now," Yang told reporters after a round featuring birdies on each of the four par-three holes.
"I think that my conservative approach, just trying to hit the greens, has been helpful. I try to make pars, less bogeys, and I've been lucky enough to make a few birdies."
British Open champion Louis Oosthuizen of South Africa and American Ryan Palmer opened with 69s. Holder Graeme McDowell of Britain, Americans Stewart Cink, Davis Love III and Chez Reavie and Swedes Johan Edfors and Henrik Stenson returned 70s.
However the heavyweight trio of Luke Donald, Lee Westwood and Martin Kaymer -- the world's top three golfers -- failed to spark after playing together in the first round.
World number one Donald of Britain shot a three-over 74, second-ranked compatriot Westwood posted a 75 and PGA champion Kaymer of Germany registered a 74.
"It's always fun playing with those guys, they're friends," Donald told Reuters. "But we all struggled a bit and we couldn't get much momentum going. Hopefully it will be different tomorrow."
With Congressional's greens fairly receptive after being softened by morning rain, scoring was overall better than expected in the early going at the year's second major.
"They (the greens) were still pretty firm, but the ball would stop," 2009 British Open champion Cink said after mixing four birdies with three bogeys.
"Just a little bit of moisture in the grass makes a huge difference on a course like this. We had a little rain in the morning and it delayed the drying out process until probably about now.
"And if the wind keeps up, we had it about as good as it's going to get," added Cink, who ended his round as the wind began to strengthen at tree-lined Congressional.
Oosthuizen, who landed his maiden major title with a crushing seven-stroke victory at last year's British Open, was pleasantly surprised by the scoring conditions.
"Everyone expected it to be a bit quicker, the greens, but they're good," the South African said after recovering from a bogey-bogey start.
"They're running really nice, and you're still getting a lot of good putts out there. There are birdies out there, but if you hit one or two wrong, you can so easily make a bogey."
Oosthuizen was in an upbeat mood after clawing his way into contention following his poor start.
"I feel confident with the game," he said after recording six birdies in his last 15 holes. "It's just a matter of going out and getting the job done.
"I didn't have a good start, but I had a good comeback after that. I'm happy with my score."
Northern Irishman McDowell, who claimed his first major title by one shot at Pebble Beach last year, was also delighted with his start on the second longest course to stage a U.S. Open.
"I set myself some challenges this morning to go out and try to think well, try to go through my processes correctly, through my routines, and just be patient and enjoy the round," he said.
"And I really accomplished those things. I played smart golf. My iron play could have been better, but all in all, very, very happy with most departments today."
Phil Mickelson, the American world number five who has been a U.S. Open runner-up five times, and Masters champion Charl Schwartzel of South Africa were among the day's late starters.
Early morning drizzle gave way to warm sunshine and light breezes at Congressional where showers and thunderstorms are forecast for later in the day.